This invention relates to aqueous printing inks and, more specifically, to aqueous pigmented ink jet ink formulations suitable for use in commercial ink jet ink printers. The most popular form of ink jet ink printing is a type of digital printing in which droplets of ink are ejected on demand from an orifice in a pinhead chamber in response to pressure increases in the chamber. The amplitude and frequency of the chamber pressure profiles are controlled digitally through a computer by sending electronic signals which results either in localized heating and boiling of the ink, or which activates a piezoelectric vibrating crystal in the chamber.
The past decade has seen a dramatic increase in this type of printing for office and personal printer applications primarily due to its relatively low cost, flexibility, speed, and quiet operation. Recent years have seen an even more dramatic increase in the popularity of ink jet printers due to the further development and resulting introduction of high quality printers, software, media and improved inks all of which tend to produce near photographic quality color graphics. Despite this rapid rise in the popularity of ink jet printing, a problem, which has persisted for photographic applications, is the lack of wet rub resistance and the instability of color quality to UV exposure. This problem is related to the fact that the ink in an ink jet head performs much more reliably as a single-phase system, i.e. where the colorant is a dye dissolved in the liquid phase. An additional problem is that the liquid phase is traditionally water based due to environmental concerns, and water soluble dyes typically exhibit less stability to UV radiation compared to organic soluble dyes, and much less stability compared to colored organic pigments. The fact that a two phase system, i.e. a pigment dispersion, in an ink jet ink presents performance problems also has a negative impact for proofing applications for traditional printing processes. The fact is that this large industry which requires reliable proofing for customer acceptance of printing jobs prior to the actual printing traditionally uses pigmented printing inks because of their color stability, and the color value of pigments are distinctly different compared to the color value of dyes.
There can be problems associated with the formulation of ink jet inks, which are required to perform consistently in an ink jet printer. The introduction of pigments in an ink jet ink can lead to additional problems of 1) clogging of the ink jet head orifice; and 2) sedimentation of pigment particles in the ink cartridge or the ink jet head. Since a pigment dispersion which is professionally formulated and dispersed is typically 50 to 100 times smaller than the ink jet head orifice, clogging is usually not the problem unless pigment sedimentation occurs in the ink jet head. Sedimentation of pigment particles in the ink cartridge results in a colorant concentration gradient that leads to inconsistent print performance with respect to color value. The fact is that a water-based ink has a density close to 1 g/cc compared to colored organic pigment particles that have densities above 1.3 g/cc. Proper performance of an ink jet ink, furthermore, requires that the viscosity of the ink be less than 10 cp, as compared to pure water which has a viscosity of lcp and commercial paints which have viscosities above 1,000 cp. The physics of the situation is that the sedimentation rate of a particle in a liquid is directly proportional to the particle size and the difference in density between the particle and the liquid medium, and inversely proportional to the viscosity of the liquid medium. Since the liquid medium in most cases is water based with a fixed density and a low viscosity, the sedimentation rate can be reduced or eliminated only by reducing the particle size or matching the density of the particles to the liquid medium. Experimental results, furthermore, have shown that when the calculated particle sedimentation rate falls below a value of 0.05 cm/hr, the sedimentation rate approaches zero due to Brownian motion, which is a random motion induced by thermal forces greater than those imposed by gravity under these conditions. For example, it has been shown experimentally that aqueous dispersions of monodisperse polystyrene latex, where the density differential is 0.05 g/cc, will never settle for particles smaller than 0.2 microns. It should be recognized that the maximum particle size required for zero sedimentation of aqueous dispersions of commercial organic pigments, where the density differential is 0.3 g/cc or greater, is less than 0.1 micron. It should be further recognized that the color value for pigments decrease as the particle size falls below 0.1 micron.
A review of recent developments in ink compositions and processes for formulating and making the same suggest that the primary focus of research activities has been directed towards the following objectives: 1) manufacturing organic pigment particles with smaller primary particle size, where a primary particle is defined as the smallest subdivision of particles; 2) utilizing dispersion techniques which result in pigment dispersions consisting primarily of primary particles, i.e. achieving the smallest particle size distribution possible; and 3) formulating the particle dispersions with surfactants which will stabilize the primary particles with respect to flocculation which leads to a larger particle size.
The present invention relates to an ink composition comprising a liquid medium with an array of encapsulating particles contained in the liquid medium. The respective encapsulating particles include one or more pigment particles and wherein the encapsulating particles are of a density within approximately 10% of the density of the liquid medium. In one particular embodiment, the size of the pigment particles are predominantly less than 0.3 microns while the size of the encapsulating particles are predominantly in the range of 0 to 1 micron. Further in this particular embodiment, the net density of the encapsulating particles is in the range of 0.9-1.1 g/cc.
Further the present invention entails a process for forming or producing an ink composition. The process comprises encapsulating pigment particles having a density greater than 1.0 g/cc in a matrix having a density of less than 1.0 g/cc to form encapsulating particles having one or more pigment particles contained therein. The process further includes containing the encapsulating particles in a liquid medium, and providing the pigment particles and matrix in a ratio that yields a net density for the encapsulating particles that is within 10% of the density of the liquid medium.
In a particular process for producing the ink composition of the present invention, the process entails producing an ink composition that includes pigment particles encapsulated in a matrix where the encapsulated matrix is neutrally buoyant with respect to a liquid medium containing the encapsulated pigment particles. The process entails dispersing pigment particles in the encapsulating matrix where the encapsulating matrix has been heated to a melting point but below the boiling point of the liquid medium. Further, a surfactant is added to the pigment particle-matrix solution for maintaining dispersion stability of the pigment particles. Additionally, the liquid medium is heated to a temperature that approximates the temperature of the heated matrix. Thereafter, the process includes emulsifying the encapsulated matrix dispersion in the heated liquid medium. By emulsifying the encapsulating matrix within the liquid medium, the ink composition is formed. Thereafter, the ink composition is cooled.